Light-emitting diodes (LEDs), particularly white LEDs, have increased in size in order to provide the total light output needed for general illumination. As LED technology has advanced, the efficacy (measured in lumens/Watt) has gradually increased, such that smaller die areas now produce as much light as was previously created by emission from far larger die areas. Nonetheless, the trend favoring higher light outputs has led to larger semiconductor LED die sizes, or, for convenience, arrays of smaller die areas in series or series-parallel arrangements. Series arrangements are generally favored because the forward voltage of LEDs varies slightly, resulting, for parallel arrangements, in an uneven distribution of forward currents and, consequently, uneven light output.
For many applications, it is desirable to have a light source that produces a light beam having a variable angular distribution. Variability is needed, for example, to create a wide-angle light beam for illuminating an array of objects, or a narrow-angle beam for illuminating a single, small object. Conventionally, the angular distribution is varied by moving the light source(s) (e.g., the LED arrangement) toward or away from the focal point of a lens or parabolic mirror. As the light source is moved away from the focal point, its image is blurred, forming a wider beam. Unfortunately, in doing so, the image is degraded, becoming very non-uniform. A need, therefore, exists for light sources that produce variable beam angles with uniform illumination and without sacrificing beam quality.